U.S. patent number 7,104,380 [Application Number 10/942,071] was granted by the patent office on 2006-09-12 for dual area piston for transmission clutch and sequential control therefor.
This patent grant is currently assigned to Ford Global Technologies, LLC. Invention is credited to Pete Bezjak, Brian Bishop, Karl Jungbluth.
United States Patent |
7,104,380 |
Bishop , et al. |
September 12, 2006 |
Dual area piston for transmission clutch and sequential control
therefor
Abstract
A system for actuating a clutch that alternately driveably
connects and disconnects components, includes a clutch having a
piston that includes a first apply area and a second apply area, a
fluid pressure source, a source of variable control pressure, and a
control coupled with the fluid pressure source and operative in
response to the control pressure to engage the clutch initially by
increasing pressure steadily at the first apply area up to a first
magnitude followed by a rapid increase in pressure at the first
apply area and the second apply area above the first magnitude to a
second magnitude.
Inventors: |
Bishop; Brian (Ira Township,
MI), Bezjak; Pete (Dearborn, MI), Jungbluth; Karl
(Orchard Lake, MI) |
Assignee: |
Ford Global Technologies, LLC
(Dearborn, MI)
|
Family
ID: |
36032700 |
Appl.
No.: |
10/942,071 |
Filed: |
September 15, 2004 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20060054444 A1 |
Mar 16, 2006 |
|
Current U.S.
Class: |
192/85.32;
192/109F; 192/85.39 |
Current CPC
Class: |
F16D
25/0638 (20130101); F16D 48/02 (20130101); F16D
2048/0212 (20130101) |
Current International
Class: |
F16D
25/0638 (20060101); F16D 25/12 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bonck; Rodney H.
Attorney, Agent or Firm: Kelley; David B. MacMillan,
Sobanski & Todd, LLC
Claims
What is claimed is:
1. A system for actuating a clutch that alternately driveably
connects and disconnects components, comprising: a clutch including
a cylinder, a piston displaceable in the cylinder and including a
first apply area and a second apply area; a fluid pressure source;
a source of variable control pressure; and a control coupled with
the fluid pressure source and operative in response to the control
pressure to engagd the clutch initially by increasing pressure at
the first apply area to a first magnitude followed by an increase
in pressure at the first apply area and the second apply area above
the first magnitude to a second magnitude, and to disengage the
clutch in response to the control pressure initially by decreasing
pressure at the second apply area the second magnitude, and
decreasing pressure at the first apply area from the second
magnitude followed by a decrease in pressure at the first apply
area.
2. The system of claim 1 wherein the second apply area is larger
than the first apply area.
3. The system of claim 1 wherein the control includes an orifice
located between the fluid pressure source and the second apply
area, the orifice being sized to produce a desired rate of fluid
flow to the second apply area from the fluid pressure source.
4. The system of claim 1 further comprising: a seal located between
the first apply area and the second apply area for sealing against
passage of fluid therebetween.
5. A system of claim 1 including: a first passage communicating a
first output of the control and the first apply area; and a second
passage communicating a second output of the control and the second
apply area.
6. A system for controlling actuation of a clutch that alternately
driveably connects and disconnects components, comprising: a clutch
including a cylinder, a piston displaceable in the cylinder, a
first apply area and a second apply area formed on the piston,
which areas are pressurized to actuate the clutch; a fluid pressure
source; a source of variable control pressure; a regulator valve
communicating with the fluid pressure source, responsive to the
control pressure and a feedback pressure at the first apply area to
regulate pressure at the first apply area up a first magnitude of
control pressure; a first latch valve responsive to the control
pressure to open communication between the first apply area and
said feedback to the regulator valve while control pressure is
equal to or less than the first magnitude, and to close said
communication when control pressure exceeds the first magnitude;
and a second latch valve responsive to the control pressure to
close communication between the second apply area and the fluid
pressure source when control pressure is equal to or less than a
second magnitude, and to open said communication when control
pressure exceeds the second magnitude.
7. The system of claim 6, further comprising: an orifice located
between the fluid pressure source and the second apply area, the
orifice being sized to produce a desired rate of fluid flow to the
second apply area from the fluid pressure source.
8. The system of claim 6, wherein the regulator valve further
comprises: a chamber including a first port communication with the
fluid pressure source, a second port spaced from the first port and
communicating with exhaust pressure, and a third port communicating
with the first apply area; a spool supported in the chamber for
displacement, having a first land communicating with the control
pressure, and a second land communicating with the feedback
pressure, and various control lands spaced mutually along the
spool; a control spring producing a force applied to the second
land and opposing spool displacement due to the control pressure;
and wherein the control lands open and close communication between
the first port and the second port, and open at close communication
between the second port and third port in response to the effect on
the spool of the control spring, the control pressure and the
feedback pressure.
9. The system of claim 8, wherein the first latch valve further
comprises: a second chamber including a fourth port communication
with the first apply area, a fifth port communicating with second
control land, and a sixth port communication with an exhaust
pressure; a second spool supported in the chamber for displacement
including a third land communicating with the control pressure, and
various control lands spaced mutually along the second spool; a
second control spring producing a force opposing spool displacement
due to the control pressure; and the control lands opening and
closing communication between the fourth port and the fifth port,
and opening and closing communication between the fifth port and
sixth port in response to effect on die spool of the second control
spring, and the control pressure.
10. The system of claim 6, wherein the second latch valve further
comprises: a third chamber including a seventh port communication
with the second apply area, and an eighth port communicating with
the fluid pressure source; a third spool supported for displacement
in the third chamber including a fourth land communicating with the
control pressure, and a control land; a third control spring
producing a force opposing displacement of the third spool due to
the control pressure; and the control land opening and closing
communication between the seventh port and the eighth port in
response to the effect on the spool of the third control spring and
the control pressure.
11. The system of claim 6 further comprising a device for opening
and closing a volume of the cylinder that is bounded in part by the
second apply area in response to a differential pressure across the
device.
12. A system for controlling actuation of a clutch that alternately
driveably connects and disconnects components, comprising: a clutch
including a piston, a first apply area and a second apply area
formed on the piston, which areas are pressurized to actuate the
clutch; a fluid pressure source; a source of variable control
pressure; a regulator valve including: a chamber including a first
port communication with the fluid pressure source, a second port
spaced from the first port and communicating with exhaust pressure,
and a third port communicating with the first apply area; a spool
supported in the chamber for displacement, having a first land
communicating with the control pressure, and a second land
communicating with a feedback pressure, and various control lands
spaced mutually along the spool; a control spring producing a force
applied to the second land and opposing spool displacement due to
the control pressure; and the control lands open and close
communication between the first port and the second port, and open
and close communication between the second port and third port in
response to the effect on the spool of the control spring, the
control pressure and the feedback pressure; a first latch valve
including: a second chamber including a fourth port communication
with the first apply area, a fifth port communicating with second
control land, and a sixth port communicating with an exhaust
pressure; a second spool supported in the chamber for displacement
including a third land communicating with the control pressure, and
various control lands spaced mutually along the second spool; a
second control spring producing a force opposing spool displacement
due to the control pressure; and the control lands open and close
communication between the fourth port and the fifth port, and open
and close communication between the fifth port and sixth port in
response to the effect on the spool of the second control spring,
and the control pressure; and a second latch valve including: a
third chamber including a seventh port communication with the
second apply area, and an eighth port communicating with the fluid
pressure source; a third spool supported for displacement in the
third chamber including a fourth land communicating with the
control pressure, and a control land; and a third control spring
producing a force opposing displacement of the third spool due to
the control pressure; and the control land open and close
communication between the seventh port and the eighth port in
response to the effect on the spool of the third control spring and
the control pressure.
13. A method for controlling actuation of a clutch that includes a
piston, a first apply area and a second apply area formed on the
piston and pressurized to actuate the clutch, comprising the steps
of: providing a fluid pressure source; providing a source of
variable control pressure; using control pressure and feedback
pressure from the first apply area to regulate pressure at the
first apply area up to a first magnitude of control pressure;
opening a connection between the fluid pressure source and the
first apply area when control pressure exceeds the first magnitude;
and using control pressure to close communication between the
second apply area and the fluid pressure source when control
pressure is equal to or less than a second magnitude.
14. The method of claim 13 further comprising the step of: using
control pressure to open communication between the second apply
area and the fluid pressure source when control pressure exceeds
the second magnitude.
15. A system for actuating a clutch that alternately driveably
connects and disconnects components, comprising: a clutch including
a cylinder, a first piston displaceable in the cylinder and
including a first apply area, and a second piston displaceable in
the cylinder including a second apply area; a fluid pressure
source; a source of variable control pressure; and a control
coupled the fluid pressure source and operative in response to the
control pressure to engage the clutch initially by increasing
pressure at the first apply area to a first magnitude followed by
an increase in pressure at the first apply area and the second
apply area above the first magnitude to a second magnitude, and to
disengage the clutch in response to the control pressure initially
by decreasing pressure at the second apply area from the second
magnitude, and decreasing pressure at the first apply area from the
second magnitude followed by a steady decrease in pressure at the
first apply area.
16. The system of claim 15 wherein the control includes an orifice
located between the fluid pressure source and the second apply
area, the orifice being sized to produce a desired rate of fluid
flow to the second apply area from the fluid pressure source.
17. The system of claim 15 further comprising: first seals for
sealing the first apply area and the cylinder against passage of
fluid therebetween; and second seals for sealing the second apply
area and the cylinder against passage of fluid therebetween.
Description
BACKGROUND OF THE INVENTION
This invention relates to a friction control element, such as a
hydraulically or pneumatically actuated clutch, of the type used to
control operation of an automatic transmission. In particular, the
invention pertains to a control for producing staged engagement and
disengagement of such a clutch having at least two sealed areas on
its actuating piston.
Automatic transmissions are typically designed to transmit full
engine torque and the engine torque as amplified by a torque
converter at stall torque under static. i.e., non-shifting
conditions. The control system of an automatic transmission
includes a low/reverse clutch, which is applied or engaged to
produce the lowest forward speed ratio and the reverse drive speed
ratio. Such engagement produces a drive connection between
components of the planetary gearing, which when selectively
combined with the engagement of other control elements, results in
the transmission operating in low gear or reverse gear. When the
clutch is disengaged, another of the several forward speed ratios
can be produced upon engagement of another combination of friction
control elements. Therefore, gearshifts into and out of low gear, 1
2 upshifts and 2 1 downshifts, are produced at least in part by
engaging and disengaging, respectively, the low/reverse clutch.
Throughout this discussion, the term "friction control element"
refers to a hydraulically actuated friction clutch or brake of a
control system.
In order for the transmission to have the static torque capacity
required to hold full stall torque, the low/reverse clutch is
typically designed with a high gain to provide the required torque
capacity to the low/reverse clutch. This high gain requirement,
however, can affect good shift quality.
In a fully synchronous automatic transmission, all the gear ratio
changes occur by coordinating the simultaneous disengagement and
engagement of two friction control elements. In a fully synchronous
automatic transmission, the low/reverse clutch controls 2 1
downshift events using a low gain clutch. In order to meet the
shift quality requirements for all 2 1 events as well as to provide
the static capacity required to hold stall torque, a low/reverse
clutch must have at least two magnitudes of gain. A clutch having
only a single gain will not suffice.
A clutch can produce multiple gains by providing multiple pressure
areas on the hydraulic piston that actuates the clutch, primary and
secondary pressurized areas. Production automatic transmissions
have used this design technique in combination with control of the
secondary pressure area on the actuating piston through operation
of the transmission manual valve. This approach merely pressurizes
both piston areas based on manual valve position with some degree
of hydraulic control.
There is a need to provide direct control of the secondary area,
preferably under control of an electronic control module and a
pressure control device. This need is especially acute for a
synchronous transmissions.
This invention provides direct control when the secondary area
applies and is controlled via the electronic control module and a
pressure control device. This allows the dual area clutch design to
be used for shift events in synchronous transmissions.
SUMMARY OF THE INVENTION
The invention relates to a single low/reverse clutch piston with
two distinct areas, which create distinct static and dynamic clutch
gains. This invention provides direct control of the pressurized
state of the secondary piston area, the application and control of
pressure in the clutch via an electronic control module and a
pressure control device. This allows the dual area clutch design to
be used for shift events in synchronous transmissions.
The clutch and control system of this invention produce very fast
response times, low dynamic gain for excellent shift quality and
high static capacity for high torque applications.
A control according to this invention uses dual valve trains to
control application of each element and allows tuning of the
response of each portion of the piston. The sequential nature of
the operation of the clutch also reduces any excessive load on the
hydraulic system of the transmission, thereby eliminating any
capacity drops, and the resulting clutch slip, during application
of pressure to the static area of the clutch piston.
The clutch design is combined with a control system that uses the
smaller dynamic piston area to stroke the clutch and conduct the
shift event. After the dynamic event is complete, the control
system seats or closes a check ball located behind the secondary
piston area and then pressurizes the secondary piston area to
provide the added capacity required for static events. The clutch
and control system use a single pressure control device and a valve
train for each portion of the dual area piston to control
activation of the clutch. The dynamic low gain portion of the
piston has an optimized small volume to react quickly, a check ball
to prevent creating a vacuum in the secondary piston volume when
stroking the piston using the secondary area and to provide a low
overall gain for excellent gearshift quality. The dynamic low gain
portion of the piston has an optimized small volume to react
quickly and provide a low overall gain for excellent gearshift
quality. The secondary area is controlled via the same pressure
control device as the first area, but uses its own valve train to
determine when to apply. Once in static capacity mode with both
piston areas applied, the clutch gain is high for static capacity
purposes. The release of the clutch is also coordinated. The
larger, static piston area is dumped quickly, while the shift event
takes place on the smaller low gain portion of the piston.
A system according to this invention for actuating a clutch that
alternately driveably connects and disconnects components, includes
a clutch having a piston that includes a first apply area and a
second apply area, a fluid pressure source, a source of variable
control pressure, and a control coupled with the fluid pressure
source and operative in response to the control pressure to engage
the clutch initially by increasing pressure steadily at the first
apply area up to a first magnitude followed by a rapid increase in
pressure at the first apply area and the second apply area above
the first magnitude to a second magnitude.
Various objects and advantages of this invention will become
apparent to those skilled in the art from the following detailed
description of the preferred embodiment, when read in light of the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross section taken at a diametric plane through a
hydraulically actuated friction clutch of an automatic transmission
whose piston has a static area and a dynamic area;
FIG. 2 is a schematic diagram of a system for controlling
engagement and disengagement of the clutch by sequentially
pressurizing the control areas of the clutch piston;
FIG. 3 is a graph showing the variation of D1 (primary area) clutch
apply pressure and D2 (secondary area) clutch release pressure vs.
the magnitude of commanded current applied to the variable force
solenoid of FIG. 2 by a transmission control unit; and
FIG. 4 is a cross section taken at a diametric plane through a
friction clutch having nested pistons for actuating the clutch.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, there is illustrated in FIG. 1 a
hydraulically actuated friction clutch 10, preferably the
low/reverse clutch of an automatic transmission, which is located
in a transmission housing. A connecting member 12, secured to and
rotating with a component of a planetary gear set, having an inner
surface on which spline teeth 14, directed parallel to an axis 16,
are formed. The clutch is arranged substantially symmetrically
about axis 16. Pressure plates 18, spaced mutually along the axis
16, have teeth 20 located at a radially outer periphery and
engaging the spline teeth 14. Located between each pressure plate
18 is a clutch disc 22 having teeth 24 located at a radially inner
periphery and engaging axially directed spline teeth 26 formed on a
member connecting 28, which is secured to and rotates with another
component of a planetary gearset. A backing plate 30, similarly
splined to the internal splines 14, is secure to the housing
against displacement. As is conventional, each discs 22 carries
friction material, which contacts and frictionally engages the
adjacent pressure plate when the clutch 10 is applied. In this way,
the clutch alternately driveably connects and releases the
components secured to connecting members 12 and 28.
A piston 34 is supported on a hydraulic cylinder 36 for axial
displacement relative to the discs 22 and pressure plates 18. The
piston is sealed on the cylinder preferably by O-rings 38, 40, 42
or another type of dynamic seal, against the passage of hydraulic
fluid. The seals, divide the piston into two hydraulically
separated zones. A primary, dynamic piston surface area 44 is
located in one zone between seals 38 and 40; a secondary static
piston surface area 46 is located in the other zone between seals
40 and 42. A check ball 41, located behind the piston area 46,
opens to admit air into the cylinder space adjacent the secondary,
static piston area 46 when piston 34 is displaced by pressure
applied to the primary, dynamic piston area 44 area. This opening
through the check valve 41 prevents a vacuum from forming in that
portion of the cylinder as the piston moves in response to DI
pressure. The check valve seats and closes when hydraulic pressure
is applied to piston area 46. As an alternative to the check valve
41, any suitable device, such as a dynamic seal that responds to a
pressure differential, can be used for this purpose.
A return spring 50, preferably a Belleville spring, is resiliently
preloaded in contact with a snap ring 52, which is secured in a
groove 54 on the cylinder 36, and with the piston 34. A force
developed in the spring 50, as the piston moves rightward from the
position of FIG. 1, opposes such displacement and tends to return
the piston to the disengaged position of FIG. 1.
The piston is displaced rightward to engage the clutch when
hydraulic pressure is applied to one or both of the spaces between
the cylinder piston areas 44 and 46. Before fully engaging the
clutch, the clutch is first stroked by applying regulated pressure
to the primary area 44, thereby taking up clearances between clutch
components principally spaces between the clutch discs and pressure
plates. Preferably, the stroke displacement of the clutch is
performed with close control so that it is completed without excess
displacement or pressure. After the clutch is stroked, the clutch
becomes fully engaged by applying pressure to the secondary piston
area 46. The clutch must have torque capacity sufficient to produce
and hold a force between the pressure plates 18 and discs 22 such
that the clutch can transmit between the connecting members 12 and
14 the magnitude of torque required in the oncoming gear ratio.
FIG. 2 illustrates a system 60 for controlling the staged
application of hydraulic pressures and fluid flow, which first
stroke and then fully engage clutch 10. The system 60 includes a
valve controlled by a variable force solenoid (VFS) 62 that
responds to a command signal produced by an electronic transmission
control unit 63 (TCU), which controls operation of the transmission
and its gear ratio changes. The VFS 62 controls a hydraulic valve,
whose output pressure varies inversely with the magnitude of
electric current supplied to the VFS 62. In the non-limiting
example discussed here, the current control signal applied to VFS
62 varies in the range 850 50 mA. In response to the control
current, the VFS-controlled valve produces pressure, which is
applied to the end surface of a land on each of a D1 regulator
valve 64, a D1 latch valve 66, and a D2 latch valve 68. FIG. 3
illustrates the variation of D1 clutch apply pressure and D2 clutch
apply pressure produced by system 60 as the magnitude of the VFS
current changes.
When VFS current is in the range of about 250 675 mA, the forces on
the spool regulator valve 64 include the force of VFS pressure on
land 70, the force of spring 72 on land 76, and the force of D1
feedback pressure on land 76. These forces regulate D1 pressure at
clutch area 44 causing it to increase linearly and inversely with
VFS current while VFS current is between about 250 mA and 675 mA,
as illustrated in FIG. 3. Subject to these forces, regulator valve
64 alternately increases the magnitude of D1 pressure by opening a
connection between line pressure feed 78 and line 80 and closing
exhaust port 82 to line 80 when the spool of the valve moves
upward, and decreases the magnitude of D1 pressure by closing a
connection between line pressure feed 78 and line 80 and opening
exhaust port 82 to line 80 when the spool of valve 64 moves
downward.
D1 latch valve 66 has potential both to control D1 feedback
pressure and to have no control over feedback pressure in line 74,
depending on the magnitude of VFS current and VFS pressure. When
VFS current is greater than about 250 nA and VFS pressure is
relatively low, land 84 opens a connection between D1 feedback line
74 and line 86, which communicates with D1 area 44. When VFS
current is equal to or less than about 250 mA, VFS pressure forces
spool 88 of the D1 latch valve 66 rightward against the force of
control spring 90, thereby closing line 86 and opening a connection
between feedback line 74 and exhaust port 92. This eliminates
feedback regulation of D1 regulating valve 64 and fully opens line
pressure feed 78 to D1 area 44. FIG. 3 illustrates the step
increase in D1 clutch apply pressure carried to area 44 through
line 80 when VFS current reaches its latching pressure current.
The D2 latch valve 68 is continually connected to VFS pressure,
which is applied to land 94. An orificed line pressure feed line 96
connects line pressure to D2 latch valve 68 through an orifice 98,
which is sized to produce a desired flow rate of hydraulic fluid to
D2 area 46. When pressure is applied to the D2 area 46, that
pressure seats the check ball 41 located behind piston 34, thereby
sealing the area 46 and allowing pressure to build in the D2
volume. That flow rate is preferably established such that the
relatively large volume of fluid required to fill area 46 does not
exceed the capacity of the transmission pump required to supply
adequately other portions of the transmission hydraulic
circuit.
When VFS current is about 250 mA, pressure on land 94 forces the
spool 98 of D2 latch valve 68 upward against the force of spring
100, thereby allowing land 102 to open a connection between
orificed line pressure feed line 96 and line 104, through which D2
clutch area 46 is filled with fluid and pressurized at a rate
determined by the size of orifice 98. The VFS current and the
corresponding VFS pressure at which D1 and D2 are latched may be
substantially equal. The clutch torque capacity continues to
increase until the commanded VFS current reaches about 70 mA and
pressure at D1 area 44 and D2 area 46 are about 15.5 bar.
The clutch disengages in response to VFS pressure increasing to 250
mA, which delatches the latch valves 66, 68 allowing the D2 volume
to drain through line 104 and exhaust port 106, and the check ball
41 then opens to atmospheric pressure. As VFS pressure declines, D1
latch valve 66 again controls feedback pressure in line 74, thereby
linearly reducing D1 pressure until VFS current increases to about
850 mA.
In this way the clutch is engaged and disengaged in stages. First
during an early, dynamic phase of clutch engagement, the clutch is
quickly stroked with low gain control producing linearly increasing
D1 pressure that is applied to the relatively small D1 area 44 and
the corresponding clutch cylinder volume. After the dynamic phase,
the area D1 44 is rapidly pressurized to line pressure. The full
torque capacity of the clutch is developed upon filling and
pressurizing the relatively large D2 area 46 and its corresponding
clutch cylinder volume with fluid from a source of line pressure
through orifice 98. Both D1 area 44 and D2 area 46 are pressurized
at relatively high pressure, during the static phase of clutch
engagement.
FIG. 4 is a cross section of a clutch 110 for use with a system
according to this invention, the clutch including nested actuating
pistons 112, 114, displaceable in a hydraulic cylinder 116, rather
than a single piston. The first piston 112 is sealed at the
cylinder surface by O-rings 118, 120, or another type of dynamic
seal, against the passage of hydraulic fluid, the seals 118, 120
providing a boundary for a primary, dynamic pressure area 122 on
the face of the piston 112 between the seals. The second piston 114
is sealed at the cylinder surface by O-rings 124, 126, against the
passage of hydraulic fluid, the seals 124, 126 providing a boundary
for a secondary, static pressure area 128 on the face of the piston
114 between those seals.
The pistons 112, 114 are actuated by hydraulic pressure supplied
through lines (not shown) connected to the outputs of the system of
FIG. 2, i.e., clutch areas D1 and D2. Piston 112 moves rightward to
engage the clutch in response to hydraulic pressure applied to the
clutch area 122 (D1). Before the clutch 110 is fully engaged, the
clutch is first stroked by applying pressure to the primary area
122, thereby taking up clearances between clutch components,
principally spaces between the clutch discs and pressure plates 20,
24. Preferably, the stroke displacement of the clutch is performed
with close control so that it is completed without excess
displacement or pressure. After the clutch is stroked, the clutch
becomes fully engaged by applying pressure to the secondary piston
area 46. The clutch must have torque capacity sufficient to produce
and hold a force between the pressure plates 18 and discs 22 such
that the clutch can transmit between the connecting members 12 and
14 the magnitude of torque required in the oncoming gear ratio.
A check ball 41, located behind the piston area 46, opens to admit
air into the cylinder space adjacent the secondary, static piston
area 46 when piston 34 is displaced by pressure applied to the
primary, dynamic piston area 44 area. This opening through the
check valve 41 prevents a vacuum from forming in that portion of
the cylinder as the piston moves in response to DI pressure. The
check valve seats and closes when hydraulic pressure is applied to
piston area 46.
The piston is actuated for rightward displacement to engage the
clutch when hydraulic pressure is applied to one or both of the
spaces between the cylinder piston areas 44 and 46. Before fully
engaging the clutch 110, the clutch is first stroked by applying
regulated pressure to the primary area 122, the D1 area, thereby
taking up clearances between clutch components, principally spaces
between the clutch discs and pressure plates 18, 22. After the
clutch 110 is stroked, the clutch becomes fully engaged by applying
pressure to the secondary piston area 128, the D2 area. The force
applied by hydraulic pressure to secondary piston 114 adds to the
force applied to primary piston 112 because the pistons are in
mutual contact at both extremities of their travel in the cylinder
116. Therefore when both pressure areas, both when the clutch is
disengaged as shown in FIG. 4, and by the
In accordance with the provisions of the patent statutes, the
principle and mode of operation of this invention have been
explained and illustrated in its preferred embodiment. However, it
must be understood that this invention may be practiced otherwise
than as specifically explained and illustrated without departing
from its spirit or scope.
* * * * *